Abrasive grinding wheel and related production method

ABSTRACT

An abrasive grinding wheel ( 10 ) that comprises an annular body ( 20 ) made of abrasive material provided with a central hole ( 21 ) and a reducing ring ( 30 ) provided with a central attachment hole ( 32 ), wherein the reducing ring ( 30 ) is inserted into the central hole ( 21 ) so that the attachment hole ( 32 ) of the reducing ring ( 30 ) is coaxial with respect to the central hole ( 21 ) of the annular body ( 20 ), wherein the reducing ring ( 30 ) comprises two opposing enlarged axial end stretches ( 311  and  312 ) configured to axially lock the reducing ring ( 30 ) to the central hole ( 21 ) of the annular body ( 20 ).

TECHNICAL FIELD

The present invention relates to an abrasive grinding wheel, e.g. with adepressed or flat centre, or a cutting disc, an emery or cuttinggrinding wheel, and a method for producing it.

PRIOR ART

As is known, a grinding wheel is essentially comprised of a disc-shapedbody made of an abrasive mixture bound by a resin, reinforced byreinforcements constituted by one or more fabric nets, by one or twometallic annular elements, commonly known as washers or ring nuts, whichdelimit the hole for attaching the grinding wheel to the shaft of thecutting machine, and by an optional label made of paper or other appliedmaterial commonly used, which adheres to one of the two faces of thegrinding wheel (usually to the back one).

In the manufacture and sale of the resinoid bonded abrasive grindingwheels, e.g. of cutting discs, a significant difficulty has always beenrepresented by the remarkable number of attachment holes adopted for thevarious needs of manufacturers of cutting machines. In particular, it isfound that about four or five quality types of grinding wheels can beformed in various formats, e.g. with outer diameters of 300, 350, 400and 500 mm. Each of such grinding wheels can then be configured so as tohave at least seven different attachment hole diameters. Obviously, forthe manufacturer and for business, this situation determines at leastthirty-five different configuration possibilities of the cutting discsto be produced to fulfil the market needs, in addition to the cost forthe manufacturer of having to continuously replace in the forming mouldsof such cutting discs, the pins that create the central holes forfollowing order requests sometimes for just a few dozen items.

Furthermore, this is added to the need for the manufacturer to storereinforcement nets and metal washers with various central holes.

This is why for a long time grinding wheel manufacturers have tried toremedy this problem by supplying washers for adapting the attachmenthole made of plastic materials or aluminium included in various sizes inthe packets of discs. However, unfortunately this solution, which isalso expensive, has the problem of the poor stability of theseadaptation washers once assembled on the grinding wheel. This poorstability often creates problems in the assembly operations of cuttingdiscs on the machines. These solutions are also often totally rejectedby the users and the retailers.

Other known solutions over past times have envisaged valid and stableinsertion systems of washers, especially plastic, in the centre of theattachment holes of the cutting discs, for the adherence inside the holeof a crown of plastic material injected hot with special moulds and withinjection machinery.

This is certainly an optimal solution from the mechanical perspective,but is very expensive in terms of the systems (moulds) needed forproducing it, and not very versatile according to market requirementsas, in order to be implemented, it requires substantial orders in orderto be made cost-effective and advantageous for the manufacturer.

An object of the present invention is to overcome the mentioneddrawbacks of the prior art, within the context of a simple and rationalsolution and at a contained cost.

Such purposes are accomplished by the characteristics of the inventiongiven in the independent claims. The dependent claims outline preferredand/or particularly advantageous aspects of the invention.

DISCLOSURE OF THE INVENTION

The invention, particularly, makes available an abrasive grinding wheelthat comprises an annular body made of abrasive material provided with acentral hole and a reducing ring provided with a central attachmenthole, wherein the reducing ring is inserted into the central hole sothat the attachment hole of the reducing ring is coaxial with respect tothe central hole of the annular body, wherein the reducing ringcomprises two opposing enlarged axial end stretches (of which at leastone of the two is obtained by plastic deformation for example byriveting) configured to axially lock the reducing ring to the centralhole of the annular body. Thanks to such solution, the drawbacks of theprior art are overcome making available an abrasive grinding wheel, i.e.a cutting disc, with a reducing ring firmly fixed to the abrasiveannular body of the grinding wheel itself which can initially have asingle attachment hole for all the formats (outer diameters) and types(composition of the abrasive mixtures) in which the grinding wheel canbe made, which can then be easily and effectively adapted according tothe specific customer requirements and requests.

Advantageously, the central hole of the annular body can comprise twoopposing axial end stretches having enlarged sections with respect to acentral stretch having a tapered section axially interposed between thetwo axial end stretches having enlarged sections, the opposing axial endstretches of the reducing ring substantially engaging to measure withthe axial end stretches of the central hole of the annular body.

Again, according to an advantageous aspect of the invention, thereducing ring can be made of a plastic material, e.g. thermosetting.

A further aspect of the invention makes available a method for producingabrasive grinding wheels that comprises:

-   -   forming, through a pressing of an abrasive mixture comprising        abrasive powders and a resinoid binder, an annular body provided        with a central hole;    -   inserting a reducing ring provided with a central attachment        hole inside said central hole of the annular body, so that the        attachment hole is coaxial with respect to the central hole; and    -   fixing the reducing ring to the annular body by means of        riveting the reducing ring.

Thanks to such solution a quick, economically advantageous (compared tothe known methods), safe and repeatable method is made available forproducing an abrasive grinding wheel (or a cutting disc) with a reducingring placed inside it. Advantageously, the fixing of the reducing ringby means of riveting can comprise plastically deforming at least oneedge proximal to an axial end of the reducing ring.

Yet another aspect of the invention can provide for the formation of theannular body comprising the creation of the central hole with an axiallyvariable cross section.

Within the context of such aspect, the realisation of the central holecan provide for the formation of two opposing end stretches havingenlarged sections at two axial ends of the central hole and at least onecentral stretch having a tapered section axially interposed between thetwo end stretches.

Furthermore, the insertion of the reducing ring inside the central holecan provide for the coaxial insertion of the reducing ring in thecentral hole, so that an enlarged axial end stretch of the reducing ringsubstantially engages to measure with one of the end stretches of thecentral hole and, wherein the fixing of the reducing ring to the annularbody through riveting of the reducing ring provides for riveting afurther axial end stretch of the reducing ring so as to radially enlargesuch further axial end stretch of the reducing ring; the further axialend stretch of the reducing ring thus radially enlarged engaging tomeasure with the other end stretch of the central hole.

According to a further aspect of the invention, the fixing of thereducing ring to the annular body can be performed through hot rivetingof the reducing ring.

In this way the riveting of the reducing ring is substantiallyirreversible and the reducing ring is firmly fixed to the inside of theabrasive annular body of the grinding wheel.

According to an advantageous aspect of the invention, the productionmethod can provide for the step of enlargement through milling of theattachment hole of the reducing ring.

In this way, the abrasive grinding wheel (or cutting disc), e.g.previously stored by the manufacturer or the retailer in a format thatis universal so to speak, wherein the attachment hole has a standarddiameter regardless of the outer diameter of the abrasive annular bodyof the grinding wheel and regardless of the composition of the abrasivemixture, can then be customised according to the customer's specificrequirements in a short time and also advantageously for small ordersfrom the customer or small production batches.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will becomeclear from reading the following description provided as a non-limitingexample, with the help of the figures illustrated in the attachedtables.

FIG. 1 is a front view of an abrasive grinding wheel according to theinvention.

FIG. 2 is a sectional view along the trace of section II-II of FIG. 1.

FIG. 3 is a central sectional view of a first variant of a firstembodiment of an annular body of an abrasive grinding wheel according tothe invention.

FIG. 4 is a central sectional view of a second variant of the firstembodiment of an annular body of an abrasive grinding wheel according tothe invention.

FIG. 5 is a central sectional view of a first variant of a secondembodiment of an annular body of an abrasive grinding wheel according tothe invention.

FIG. 6 is a central sectional view of a second variant of the secondembodiment of an annular body of an abrasive grinding wheel according tothe invention.

FIG. 7 is a central sectional view of a first embodiment of a reducingring of an abrasive grinding wheel according to the invention.

FIG. 8 is a central sectional view of a second embodiment of a reducingring of an abrasive grinding wheel according to the invention.

FIGS. 9a-9f are a schematic sequence of a production method of anabrasive grinding wheel according to the invention.

BEST WAY TO ACTUATE THE INVENTION

With particular reference to such figures, 10 indicates overall anabrasive grinding wheel, e.g. of the cutting disc type, i.e. a flatabrasive grinding wheel or with a depressed centre.

The grinding wheel 10 comprises an annular body 20, provided with acentral axis A, which annular body 20 is for example substantially flat(see figures) or with a depressed centre (not shown).

The annular body 20 is substantially circular and is provided with acentral hole 21 that is coaxial with respect to the annular body 20,passing through in an axial direction from one side to the other of theannular body 20.

The annular body 20 has an outer diameter substantially comprisedbetween 250 mm and 500 mm, e.g. equal to 250 mm or 300 mm or 400 mm or500 mm.

The annular body 20 comprises two opposing faces 22 and 23, of which onefront face 22 and one rear opposing face 23, which are, for exampleparallel to each other overall and orthogonal to the central axis A ofthe annular body 20.

The annular body 20 has a substantially layered structure and has atleast one or more layers of abrasive mixture.

Each layer of abrasive mixture (once pressed and fired) defines asubstantially monolithic body.

For example, each layer of abrasive mixture is made of a mixture ofabrasive powders that is compacted and firmly bound to a binding resin.

In practice, the layer of abrasive mixture is obtained through pressinga mixture of a loose powder of abrasive material, e.g. abrasive materialsuch as natural corundum, sand, recovered artificial corundum or thelike, sol-gel abrasives or sintered ceramics, zirconium corundums, orthe like, and mixed with an appropriate binder, e.g. based on bindingresins, for example phenolic resins, liquid resins and/or powderedresins, and possibly modified with epoxy, phenoxy and/or other resins,modified with organic and/or vegetable or synthetic compounds, and othertypes of polyimide resin etc., and/or with additives and fillers.

For example, the quantity of resin is comprised between 15% and 20% byweight with respect to the weight of the abrasive material powdermixture.

The abrasive material of the layer of abrasive mixture has particle sizesubstantially comprised between 120 and 12 in mesh (however, the use ofabrasive mixtures with a larger or smaller particle size than the range,according to requirements, is not excluded).

The annular body 20 comprises one or more reinforcement nets eachsubstantially incorporated into the layer of abrasive mixture orinterposed between them.

Each reinforcement net is substantially annular and has a hole in thecentre with a diameter greater than or equal to the (minimum) diameterof the central hole 21 of the annular body 20.

In practice, the layer of abrasive mixture (or the layers of abrasivemixture) surround(s), in particular axially, the entire (lower andupper) surface of the reinforcement net.

Furthermore, the layer of abrasive mixture can also incorporate morethan one reinforcement net.

The central hole 21 of the annular body 20 preferably has an axiallyvariable cross section, e.g. variable in stretches.

In particular, the central hole 21 of the annular body 20 comprises afirst stretch 210, for example with a circular cross section (i.e.orthogonal to the central axis A) wherein such first stretch 210 has afirst diameter d1 that is the minimum diameter of the central hole 21.

The central hole 21 of the annular body 20 then comprises a secondstretch 211, e.g. with a circular cross section, wherein such secondstretch 211 has a larger second diameter d2 than the first diameter d1of the first stretch 210 (e.g. equal to the maximum diameter of thecentral hole 21).

Again, the central hole 21 of the annular body 20 then comprises a thirdstretch 212, e.g. with a circular cross section, wherein such thirdstretch 212 has a larger third diameter d3 than the first diameter d1 ofthe first stretch 210, preferably but not necessarily equal to thesecond diameter of the second stretch 211 (e.g. equal to the maximumdiameter of the central hole 21).

For example, the first diameter d1 of the first stretch 210 ispredefined, i.e. it is a fixed diameter regardless of the outer diameterof the annular body 20 and, preferably, equal for all the annular bodies20 even with different outer diameters.

Even more preferably, the first diameter d1 of the first stretch 210 isgreater than the most common inner diameters of the standard attachmentholes present on the market for such type of grinding wheels (cuttingdiscs), e.g. it is greater than 35 mm, e.g. it is equal to 40 mm.

The second diameter d2 and the third diameter d3 can be calculatedaccording to the first diameter d2 and the thickness of the annular body20 (or the axial length of the central hole 21), e.g. through thefollowing relationship:

d1=d2=d1+k;

where k is a whole submultiple of the thickness of the annular body 20(or the axial length of the central hole 21), e.g. it is equal to halfthe length of the annular body 20 or a third of the thickness of theannular body 20.

Preferably, the first stretch 210 is (central or) axially interposedbetween the second stretch 211 and the third stretch 212.

In that case, the second stretch 211 and the third stretch 212 are axialend stretches of the central hole 21 and lead directly to a respectiveface 22 and 23 of the annular body 20.

In practice, the second stretch 211 and the third stretch 212 defineopposing axial ends with an enlarged section of the central hole 21 withrespect to the tapered section axially interposed between them definedby the first stretch 210.

In a first embodiment shown in FIGS. 3 and 4, the central hole 21 has arounded profile, with a rounding radius R as a function of the thicknessof the grinding wheel 10, or a function of the axial length of thecentral hole 21.

In practice, the first stretch 210 is defined by the minimum section ofthe central hole 21 placed on the median plane of the grinding wheel 10(or of the central hole 21) orthogonal to the central axis A and isconnected with two respective substantially truncated cone (rounded)surfaces, that define the second stretch 211 and the third stretch 212,at the opposite ends of the central hole 21, with the maximum section.

In a second embodiment shown in FIGS. 5 and 6, the central hole 21 has asubstantially cylindrical shaped first central stretch 210, e.g. with aheight substantially equal to half the thickness of the grinding wheel10, i.e. equal to half the axial length of the central hole 21.

The first stretch 210 is then connected with two respectivesubstantially truncated cone shaped surfaces (substantially inclined by45° with respect to the central axis A), which define the second stretch211 and the third stretch 212, at the opposite ends of the central hole21, with the maximum section.

Finally, the grinding wheel 10—as shown in FIGS. 4 and 6—can compriseone or more thinner metal annular elements, commonly known as washers 24or ring nuts, that delimit the central hole 21 tracing the shapethereof, and act as reinforcement for it (for the contact with the shaftof the cutting machine).

The washer 24 is fixed to the rear face 23 of the annular body 20 of thegrinding wheel 10, for example for a limited radial stretch of theannular body 20 itself.

The washer 24 comprises a thin hollow central shank 24 that issubstantially inserted to measure in the central hole 21 and that hasthe same shape (of the axial stretches described above) as the centralhole 21.

In practice, the hollow central shank 24 radially delimits, withoutaltering the shape thereof described above, the central hole 21.

On the rear face 23 of the annular body 20 of the grinding wheel 10,even after the forming (and firing of the grinding wheel itself, as willbe described more clearly below), a label 25 can be placed (visible onlyin FIG. 9e ), made of paper or tinfoil or similar applied material,which is substantially annular and can occupy the whole rear face 23 ofthe annular body 20 of the grinding wheel 10 or a limited radial portionthereof, e.g. an annular crown of the rear face that starts internallyfrom the outer periphery of the washer 24.

The grinding wheel 10 then comprises a reducing ring 30, which isconfigured to (substantially) reduce the through section of the centralhole 21 of the annular body 20, as will be better described below, untilreaching an inner diameter substantially equal to the outer diameter ofthe rotary shaft of the cutting machine for which the grinding wheel 10is intended.

The reducing ring 30 comprises a substantially circular disc-shaped body31 provided with a central axis B.

Preferably but not necessarily, the disc-shaped body 31 is provided witha central attachment hole 32, e.g. coaxial with respect to thedisc-shaped body 31, which passes through in the axial direction fromone side to the other side of the disc shaped body 31.

The attachment hole 32 is for example cylindrical with a constantsection along the entire axial extension thereof.

Preferably, the diameter D0 of the attachment hole 32 is smaller thanthe most common inner diameters of the standard attachment holes presenton the market for such type of grinding wheels (cutting discs), e.g. itis equal to 20 mm.

For example, the diameter D0 of the attachment hole 32 is predefined,i.e. it is a fixed diameter regardless of the outer diameter of theannular body 20 (and of the inner diameter of the central hole 21) and,preferably, it is initially the same for all the grinding wheels 10.

For example, the diameter D0 of the attachment hole 32 is substantiallycomprised between 80% and 25% of the first diameter d1 of the centralhole 21, preferably comprised between 80% and 50% of the first diameterd1 of the central hole 21, in the example, equal to about half of thefirst diameter d1 of the central hole 21.

The attachment hole 32, when the reducing ring 30 engages the centralhole 21 of the annular body 20, is coaxial to the annular body 20 (or tothe central hole 21 thereof).

The disc-shaped body 31 comprises two opposing faces 33 and 34, of whichone front face 33 and one rear opposing face 34 which are, for example,parallel to each other overall and orthogonal to the central axis B ofthe disc-shaped body 31.

The thickness (or axial length) of the disc-shaped body 31 is defined bythe distance between the faces 33 and 34 thereof, which is preferablyequal to (or slightly larger than) the thickness of the grinding wheel10, or the axial length of the central hole 21 of the annular body 20thereof.

The disc-shaped body 31 has a shaped outer jacket which, once insertedinto the central hole 21 of the annular body 20 of the grinding wheel10, is substantially complementary to the shape of the central hole 21of the annular body 20 for which it is intended, so as to define a(substantially perfect) shape connection.

The outer jacket of the disc-shaped body 31 preferably has an axiallyvariable cross section, e.g. variable in stretches.

In particular, the outer jacket of the disc-shaped body 31 comprises afirst stretch 310 for example with a circular cross section (ororthogonal to the central axis A), in which such first stretch 310 has afirst diameter D1 that is the minimum diameter of the outer jacket ofthe disc-shaped body 31 and is substantially equal (or slightly smaller)than the first (minimum) diameter d1 of the first stretch 210 of thecentral hole 21.

The outer jacket of the disc-shaped body 31 then comprises a secondstretch 311, e.g. with a circular cross section, wherein such secondstretch 311 has a second diameter D2 greater than the first diameter D1of the first stretch 310, e.g. equal to the maximum diameter of theouter jacket of the disc-shaped body 31 and is substantially equal to(or slightly smaller than) the second (maximum) diameter d2 of thesecond stretch 211 of the central hole 21.

Again, the outer jacket of the disc-shaped body 31—once it has beenfirmly inserted into the central hole 21 of the annular body 20 of thegrinding wheel 10—comprises a third stretch 312, e.g. with a circularcross section, wherein such third stretch 312 has a third diameter D3greater than the first diameter D1 of the first stretch 310, preferablybut not necessarily equal to the second diameter D2 of the secondstretch 311, e.g. equal to the maximum diameter of the outer jacket ofthe disc-shaped body 31 and is substantially equal to (or slightlysmaller than) the second (maximum) diameter d3 of the third stretch 212of the central hole 21.

Preferably, the first stretch 310 of the outer jacket of the disc-shapedbody 31 is axially interposed between the second stretch 311 and thethird stretch 312.

In that case, the second stretch 311 and the third stretch 312 are axialend stretches of the disc-shaped body 31 and are connected directly to arespective face 33 and 34 of the disc-shaped body 31 (defining at leastone end portion thereof).

In particular, the second stretch 311 defines a first enlarged axial endof the disc-shaped body 31, i.e. that enlarges in a radial direction(with respect to the central axis B), with respect to the first stretch310 and the third stretch 312 defines an opposing second enlarged axialend of the disc-shaped body 31, i.e. that enlarges in a radial direction(with respect to the central axis B), with respect to the first stretch310.

In other words, the second stretch 311 and the third stretch 312 defineopposing axial ends with an enlarged section of the disc-shaped body 31with respect to the tapered section axially interposed between themdefined by the first stretch 310.

For example, the second stretch 311 is defined by an annular crown thatprojects radially from the disc-shaped body 31.

The annular crown that defines such second stretch 311 has asubstantially conical shape whose largest base is substantially coplanarto one of the faces 33 and 34, in the example the rear face 34.

For example, the third stretch 312, once the grinding wheel 10 isdefinitively formed, is defined by a further annular crown that projectsradially from the disc-shaped body 31.

Such further annular crown that defines such third stretch 312 has asubstantially conical shape whose largest base is substantially coplanarto the other one of the faces 33 and 34, in the example the front face33.

Preferably, the annular crown that defines the second stretch 311 issymmetrical to the further annular crown that defines the third stretch312 with respect to a median plane of the disc-shaped body 31 orthogonalto the central axis B.

In a first embodiment shown in FIG. 7, the second stretch 311 and thethird stretch 312 have a rounded profile, with a rounding radius R as afunction of the thickness of the grinding wheel 10, or a function of thethickness of the disc-shaped body 31.

In practice, the first stretch 310 is defined by the minimum section ofthe outer jacket of the disc-shaped body 31 placed on the median planeof the disc-shaped body orthogonal to the central axis B and isconnected with two respective substantially truncated cone (rounded)surfaces, that define the second stretch 311 and the third stretch 312,at the opposite ends of the disc-shaped body 31, with the maximumsection.

Such conformation of the disc-shaped body is intended to be inserted inthe first embodiment into the central hole 21 of the annular body 20shown in FIGS. 3 and 4.

In a second embodiment shown in FIG. 8, the outer jacket of thedisc-shaped body 31 has a first central stretch 310 with a substantiallycylindrical shape, e.g. with a height substantially equal to half thethickness of the disc-shaped body 31. The first stretch 310 is thenconnected with two respective substantially truncated cone shapedsurfaces (substantially inclined by 45° with respect to the central axisB), which define the second stretch 311 and the third stretch 312, atthe opposite ends of the disc-shaped body 31, with the maximum section.

As will be better described below, the third stretch 312, or the annularcrown that defines it, is obtained through (irreversible) plasticdeformation of an edge 313 (see FIGS. 7 and 8) protruding from the frontface 33 of the disc-shaped body 31 in the axial direction.

For example, the third stretch 312 is obtained through (hot) riveting.

For example, the edge 313 is movable (by irreversible plasticdeformation) from an initial position, in which it projects axially fromthe front face 33 of the disc-shaped body 31 and is contained within theradial dimensions defined by the first stretch 310, to a final(definitive) position, in which it projects radially defining said thirdstretch 312.

The edge 313 is for example a circular crown for its whole extension,but could also be defined by a plurality of circular crown shapedstretches separated from each other.

In practice, when the edge 313 is in its initial position, thedisc-shaped body 31 can be axially inserted into the central hole 21 ofthe annular body 20 (on the side of the front face 33, or by firstinserting the edge 313 and then the first stretch 310, until the secondstretch 311 of the disc-shaped body 31 abuts and is housed in the thirdstretch 212 of the central hole 21).

When instead the edge 313 comes into its final position, it is housed(to measure) in the second stretch 211 of the central hole 21 of theannular body 20, in practice radially and/or axially contained therein,in fact retaining or being retained axially inside the central hole 21without the possibility of being axially pulled out from (and rotatingwith respect to) the annular body 20.

For example, the third stretch 312 of the disc-shaped body 31, or theedge 313 brought into its final position, is adapted to adhere to atleast one surface (portion) of the second stretch 211 of the centralhole 21 of the annular body 20, e.g. through an adhesive (preferably,but not necessarily, defined by the binding resin that constitutes theannular body 20 of the grinding wheel 10 or the material—thermallysoftened and then hardened—that constitutes the edge itself).

In a preferred embodiment the disc-shaped body 30 is made overall of aplastic material, e.g. a heat deformable plastic material, preferablynylon in some of the numerous appropriate types.

Preferably, the disc-shaped body 30 is made overall of a plasticmaterial having a higher softening temperature than the softening (ormelting) temperature of the binding resin used for realizing the annularbody 20 of the grinding wheel 10.

Alternatively, it is possible to provide for the disc-shaped body 30 tobe made overall of a different material, e.g. metal or another materialsufficiently rigid to be able to define an edge 313 adapted to beplastically deformable (due to the shape or size or compositionthereof).

In light of the above description, the method for producing a grindingwheel 10 as described above is as follows.

The method firstly envisages the step of forming an annular body 20 (seeFIG. 9a ) provided with a central hole 21, having the shape describedabove.

The step of forming the annular body 20 takes place through pressing amixture of abrasive powders and binding resin (possibly stratified withthe interposition of reinforcement meshes), as described above, e.g.through a specific forming mould (as known to a person skilled in theart).

The central hole 21 is formed with the aforementioned specifications,e.g. covered or not by the aforesaid washer 24.

The rear face 23 of the annular body 20 can be covered at leastpartially by the label 25 during the forming step, or preferably, it canbe made to adhere, through relevant adhesives, at a later time accordingto the requirements.

The method then envisages arranging at least one reducing ring 30 asdescribed above, e.g. preformed, e.g. through injection moulding oranother suitable forming method.

At the start, the reducing ring 30 is configured so that the edge 313 isin its initial position described above.

At this point, the method envisages the step of coaxially inserting (seeFIG. 9b ) the reducing ring 30 into the central hole 21 of the annularbody 20, so that the attachment hole 32 of the disc-shaped body 30 iscoaxial with respect to the central hole 21 of the annular body 20.

Such insertion step can be realized in an automated or semi-automatedway by a specific insertion machine.

Furthermore, such insertion step can be realized by axially insertingthe disc-shaped body 31 into the central hole 21 of the annular body 20on the side of the front face 33, or by first inserting the edge 313 andthen the first stretch 310, until the second stretch 311 of thedisc-shaped body 31 abuts and is housed in the third stretch 212 of thecentral hole 21.

In this position, in which the second stretch 311 of the disc-shapedbody 31 abuts and is housed in the third stretch 212 of the central hole21 the rear face 34 of the reducing ring 30 is substantially coplanar tothe rear face 23 of the annular body 20 (or slightly protrudingtherefrom, e.g. by 1 or 2 tenths of a millimetre).

Furthermore, in this position, in which the second stretch 311 of thedisc-shaped body 31 abuts and is housed in the third stretch 212 of thecentral hole 21, the edge 313 in its initial position projects axiallybeyond the central hole 21 (and beyond the plane defined by the frontface 22 of the annular body 20 of the grinding wheel 10).

At this point the method envisages the step of fixing (see FIG. 9c ) thereducing ring 30 to the annular body 20.

For example, the fixing step is performed through riveting the secondstretch 312 (or the edge 313) of the reducing ring 30; in such rivetingthe edge 313 is plastically deformed so that it is brought irreversiblyfrom its initial position to its final position.

In such final position the edge 313 prevents the reducing ring 30 beingaxially pulled out from the annular body 20.

For example, the riveting is hot riveting, e.g. it envisages heating,until softening (e.g. to a temperature comprised around 250° C.), theedge 313, so that it can—once softened—be brought into its finalposition and, in fact, occupy the annular volume interposed between thereducing ring (with the edge 313 in the initial position) and the secondstretch 211 of the central hole 21 of the annular body 20. Such hotriveting also envisages cooling (or leaving to cool) the edge 313plastically deformed in the final position, until the hardening thereof,in fact making the plastic deformation undergone by the edge 313irreversible.

The hot riveting also allows the adhesion of the third stretch 312 ofthe reducing ring 30 to the walls of the (second stretch 211) of thecentral hole 21 of the annular body 20.

Preferably, the riveting 313 is performed through a specific rivetingtool 40 depicted in FIG. 9 c.

The riveting tool 40 comprises for example a circular crown, e.g. madeof metal material, defining a contact surface, adapted to come intocontact with the edge 313, which is for example covered by non-stickmaterials to promote the detachment of the edge 313 once it has beenplastically deformed (through softening). The riveting tool 40 thencomprises, as usual, heating elements 41, such as electric resistors(e.g. provided with thermostats) adapted to heat it, e.g. the contactsurface of the riveting tool 40.

The riveting tool 40 is then axially movable with respect to thereducing ring 30, so as to be able to exert a (slight) thrust or axialpressure on the edge 313 that is translated into axial and radialdeformation of the edge 313 itself.

The riveting step can even last a few seconds.

At the end of such step, the edge 313 has been brought into the finalposition and has provided the aforesaid third stretch 312 of thedisc-shaped body 31 of the reducing ring 30, as illustrated in FIG. 9 d.

In this final position, in which the third stretch 312 of thedisc-shaped body 31 abuts and is housed (to measure) in the secondstretch 211 of the central hole 21 the front face 33 of the reducingring 30 is substantially coplanar to the rear face 22 of the annularbody 20 (or slightly protruding therefrom, e.g. by 1 or 2 tenths of amillimetre at the most).

The method also envisages the step of firing the grinding wheel 10, orthe annular body 20 thereof.

Such firing step can take place prior to the step of inserting thereducing ring 30 in the central hole 21 of the annular body 20 orfollowing the fixing step (by riveting) of the reducing ring 30 to theannular body 20.

Such firing step envisages subjecting the annular body 20 (or the entiregrinding wheel 10) to a thermal firing treatment, e.g. in specificpolymerization ovens, in which the polymerization of the binding resinis completed as it stably solidifies and retains the abrasive mixturethat constitutes the annular body 20 of the grinding wheel 10.

In practice, the annular body 20 is subjected to a thermal cycle thatenvisages the insertion thereof in an oven at a temperaturesubstantially comprised between 80° and 200° C. for a time substantiallycomprised between 1 and 50 hours, or may be fired in situ in the samepressing mould, where the latter is provided with heating means.

Such firing step, where performed following the fixing step (byriveting) of the reducing ring 30 to the annular body 20 is such as notto deform the reducing ring 30.

The grinding wheel 10 thus finished or to be finished therefore has anannular body 20 provided with a central hole 21 inside which a reducingring 30 is firmly fixed, in turn with an attachment hole 32, wherein theattachment hole 32 has the aforesaid diameter D0 (a lot smaller than theminimum diameter of the central hole 21).

The method for producing the grinding wheel 10 can, further, envisagethe step of applying (see FIG. 9e ) a label 25 (self-adhesive or glued)to the rear face of the annular body 20, unless it has been previouslyapplied during the formation of the annular body 20.

In this way the label 25, as well as the information related to thegrinding wheel 10, can bear customized information related to theretailer and/or the manufacturer or other required information. Inpractice, such solution allows the affixing of customized labels(customer brand) and the specification thereon of the instructions andcharacteristics of use of the grinding wheel 10.

The method finally envisages the step of enlarging the attachment hole32 of the reducing ring 30.

For example, it is possible to enlarge the attachment hole 32 accordingto customer requests according to requirements, or to bring it tocompatible dimensions with the diameter of the rotary shaft of thecutting machine in which the grinding wheel 10 is to be assembled.

Preferably, the step of enlarging the attachment hole 32 can beperformed through milling (see FIG. 9f ).

Such milling operation can be performed through a relevant milling tool50, which has, for example, a milling cutter with increasing diameters,in stretches.

The first diameter at the free end of the milling cutter can be equal tothe diameter D0 of the attachment hole 32 of the reducing ring 30, inthat way the coaxial insertion of the milling cutter in the attachmenthole 32 of the reducing ring 30 is guided by the cylindrical couplingbetween the first diameter and the attachment hole 32 of the reducingring 30.

By axially sinking the milling cutter into the reducing ring as far asthe desired axial position corresponding to the diameter of theattachment hole 32, the attachment hole 32 will be enlarged to thedesired dimension, e.g. 22 mm, 25 mm or 32 mm or other customizablesizes.

For example, the maximum diameter to which it is possible to enlarge theattachment hole 32 is for example less than or equal to 80% of the firstdiameter d1 of the central hole 21 of the annular body 20.

This solution therefore allows the grinding wheel 10 manufacturer toprepare and store grinding wheels 10 provided with a reducing ring 30having a smaller common attachment hole 32 for a certain type ofgrinding wheels 10 (e.g. for cutting discs); such attachment hole 32could have a diameter equal to 20 mm.

Such reducing ring 30 provided with the attachment hole 32 common to allwould be assembled on all the annular bodies 20 in the quality typevariants (e.g. 5-6 different abrasive mixtures), in fact constituting astock of grinding wheels 10 to be finished, whose finalization canenvisage only the preparation (enlargement) of the attachment hole 32,also of a few dozen pieces at a time, quickly and cheaply when an orderfrom the distributor/customer is finalized.

What has been described above could be performed, being equipped withsimple equipment even for use by distributors (if different from themanufacturer), avoiding useless and expensive storage of too manydifferent types of grinding wheels in which the only variant is theattachment hole.

The invention thus conceived can undergo numerous modifications andvariants all of which are covered by the inventive concept.

Moreover, all of the details can be replaced by other technicallyequivalent elements.

In practice, the materials used, as well as the contingent shapes andsizes, can be whatever according to the requirements without for thisreason departing from the scope of protection of the following claims.

1. A method for producing abrasive grinding wheels that comprises:forming, through a pressing of an abrasive mixture comprising abrasivepowders and a resinoid binder; an annular body provided with a centralhole; inserting a reducing ring provided with a central attachment holeinside said central hole of the annular body, so that the attachmenthole is coaxial with respect to the central hole; and fixing thereducing ring to the annular body by means of riveting the reducingring.
 2. The method according to claim 1, wherein the fixing of thereducing ring by means of riveting comprises plastically deforming atleast one edge proximal to an axial end of the reducing ring.
 3. Themethod according to claim 1, wherein the formation of the annular bodycomprises the creation of the central hole with an axially variablecross section.
 4. The method according to claim 3 wherein the creationof the central hole provides for the formation of two opposing endstretches having enlarged sections at two axial ends of the central holeand at least one central stretch having a tapered section interposedaxially between the two end stretches.
 5. The method according to claim4, wherein the insertion of the reducing ring inside the central holeprovides for the coaxial insertion of the reducing ring in the centralhole, so that an enlarged axial end stretch of the reducing ringsubstantially engages to measure with one of the end stretches of thecentral hole and wherein the fixing of the reducing ring to the annularbody through riveting of the reducing ring provides for riveting afurther axial end stretch of the reducing ring so as to radially enlargesuch further axial end stretch of the reducing ring; the further axialend stretch of the reducing ring thus radially enlarged engaging tomeasure with the other end stretch of the central hole.
 6. The methodaccording to claim 1, wherein the fixing of the reducing ring to theannular body is performed through hot riveting of the reducing ring. 7.The method according to claim 1, which comprises the step of enlargementthrough milling of the attachment hole of the reducing ring.
 8. Anabrasive grinding wheel that comprises: an annular body made of abrasivematerial, obtained by pressing of an abrasive mixture comprisingabrasive powders and a resinoid binder, wherein the annular body isprovided with a central hole; and a reducing ring provided with acentral attachment hole, wherein the reducing ring is inserted into thecentral hole so that the attachment hole of the reducing ring is coaxialwith respect to the central hole of the annular body, and wherein thereducing ring comprises two opposing enlarged axial end stretchesconfigured to axially lock the reducing ring to the central hole of theannular body.
 9. The abrasive grinding wheel according to claim 8,wherein the central hole of the annular body comprises two opposingaxial end stretches having enlarged sections with respect to a centralstretch having a tapered section axially interposed between the twoaxial end stretches having enlarged sections, the opposing axial endstretches of the reducing ring substantially engaging to measure withthe axial end stretches of the central hole of the annular body.
 10. Theabrasive grinding wheel according to claim 8, wherein the reducing ringis made of a plastic material.